Anthracycline cardiotoxicity remains an important clinical problem. Acute toxic manifestations (ECG abnormalities, dysrrhythmias, myocarditis, pericarditis, transient cardiac dysfunction) are rarely life-threatening, but the chronic or delayed cardiotoxicity (related to total cumulative dose) often culminates in congestive heart failure and death. Until recently, the delayed cardiotoxicity was expected to become fully manifested within months after the last of a series of anthracycline doses (median length of one month). However, Steinherz, et al., (1991) and Lipshultz, et al., (1991) found that the initial expression of cardiotoxicity may not occur for 4 to 20 years after completion of anthracycline therapy. Based on these studies, it now appears that a large number of asymptomatic patients are at risk for developing anthracycline-induced cardiac dysfunction. Chronic anthracycline cardiotoxicity is characterized initially by impairment of left ventricular relaxation. Eventually, systolic dysfunction occurs; the left ventricular free wall progressively thins, and the ventricle dilates. However, the mechanism of the cardiotoxicity remains unclear. Putative mediators of the toxicity have been proposed, including free radicals and anthracycline metabolites, but specific intracellular targets of such injurious agents are unknown. Recent studies have accumulated considerable evidence that the mechanism of anthracycline cardiotoxicity may relate to abnormalities of calcium metabolism. Treatment of SR calcium release channels with doxorubicin acutely in vitro in a lipid bilayer (Ondrias, et al., 1990) showed a biphasic response, first increasing open probability and then reducing open probability. Based on these and other data, the applicant has formulated the following question: Does the anthracycline doxorubicin or its metabolite doxorubicinol affect the function of the cardiac calcium release channel? The applicant will address this question using the chronic rabbit model developed by Dodd, et al., 1993. Single cardiac SR Ca release channels will be studied in planar lipid bilayers from control and anthracycline treated rabbits. The applicant will examine channels treated with doxorubicin (acute in vivo and in vitro and chronic in vivo) and doxorubicinol (acute in vitro). Isolated SR vesicles will be inserted into artificial membranes and reconstituted using a cesium-based method.